Polyamides typified by nylon 66 are widely used for apparel, industrial fibers, automobile fields, electric/electronic fields, and extrusion molded products such as films and monofilaments taking advantage of its excellent characteristics. In recent years, materials for ultra-thin molded articles which endure use in a higher-temperature atmosphere are required and, for example, the demands for polyamides made from aromatic dicarboxylic acids predominantly composed of a dicarboxylic acid are gradually increased in the uses centering on automobile uses and connector uses. Further, an increase in demand for biobase polyamides such as nylon 410 and nylon 610 is anticipated.
In general, a polyamide made from a dicarboxylic acid and a diamine is produced by undergoing the following process.
First, a dicarboxylic acid is reacted with a diamine in water to prepare an aqueous solution of a salt serving as a raw material of a polyamide. Subsequently, the aqueous solution of the salt is heated to evaporate water and condensed to a prescribed concentration to obtain a mixed solution of a lower condensate (prepolymer) and water. Thereafter, the mixed solution is usually transferred to a batch type reactor, and then heated to allow polymerization to proceed, and water remaining after concentration and condensed water produced by polymerization are evaporated to obtain a polyamide.
However, in the above-mentioned method, since the mixed solution of prepolymer and water is retained for a long time under the condition of high-temperature, the mixed solution is easily gelated and, consequently, quality of the resulting polyamide tends to deteriorate. Thus, a production method is disclosed, in which a lower condensate is taken out in a state of solid phase and kneaded in a molten state to form a prepolymer, and then the prepolymer is polymerized in a state of solid phase (e.g., JP 61-228022 A). However, in such a batch process in which melt polymerization is combined with solid polymerization, the apparatus is complicated and inconvenient maintenance work becomes a problem.
Thus, a method of producing a polyamide, in which polymerization is continuously performed by using an extruder, is disclosed (e.g., JP 2010-53359 A).
In the production method disclosed in JP '359 described above, a prepolymer under high-temperature/high-pressure is fed from a pre-polymerization tank to an extruder under ordinary pressure or lower. In that time, the prepolymer becomes susceptible to deposition due to evaporation of water in the prepolymer and a reduction in temperature involved by a pressure reduction. Further, in a structure of a pump, generally, it is often that a discharge bore is smaller than an inlet bore to stabilize discharging, and a diameter of a pump discharge side pipe leading to downstream of a pump outlet is identical to the discharge bore of the pump. Accordingly, since the pump discharge side pipe leading to downstream of a pump has a diameter smaller than the inlet bore of the pump, a part of the pipe is clogged due to the prepolymer deposition, and therefore prepolymer discharge is unstable to easily cause abnormal retention within the pump discharge side pipe. Thereby, there is a problem that a difference in heat quantity which the prepolymer receives in the pump discharge side pipe occurs and quality of the polyamide is not stable.
It could therefore be helpful to provide a method by which the discharge of a prepolymer can be stabilized and a polyamide of stable quality can be continuously produced.